1. Field of the Invention
The present invention is directed to an electrical component having high strength given stressing due to temperature change and due to surge currents, the component being composed of a ceramic sintered body having a circular, rectangular or square cross section, particularly a voltage-dependent electrical resistor (varistor) of zinc oxide material which is semiconducting due to doping and whose end faces contain solderable layers as electrodes which do not extend up to the circumferential surface thereof and to which annular current leads are centrally soldered.
2. Description of the Prior Art
An electrical component, in particular an electrical resistor having a negative temperature coefficient of resistance (NTC resistor) and comprising the afore-mentioned features where, however, particularly small dimensions should be present, namely a diameter of 1.5 mm-5 mm and a thickness in the range of 0.6 mm-2.5 mm is disclosed in the German Gebrauchsmuster No. 85 06 668 of Apr. 10, 1986, published May 22, 1986, and which largely corresponds to the European patent application No. 86102471.9. Given this NTC resistor, the current leads are annularly constructed at one end as a largely closed eyelet whose outside diameter amounts to, at most 60% of the diameter of the layers of the wafer and the annular eyelet of each current lead is centrally soldered to the layers, whereby the solder surrounds the eyelet, to the extent that the solder is limited to the region of the eyelet and the edge regions of the layers are not covered with solder.
For NTC resistors having particularly small dimensions, this embodiment achieves the object of guaranteeing a terminal resistance having a drift of less than 1% for the current leads soldered to the solderable metallic layers given a temperature shock stress of 100-fold change between -60.degree. C. and 360.degree. C., preferably -36.degree. C. through +130.degree. C. and of thereby producing a stable mount of the ceramic wafer by the current leads carrying the wafer.
The prior art, particularly the German published application No. 19 47 799 and U.S. Pat. Nos. 2,606,995 and 2,686,244, fully incorporated herein by this reference, and treated in detail in the German Gebrauchsmuster No. 85 06 668, is not suitable for achieving this object, as set forth in detail therein.
In electrical components having a ceramically-manufactured body, for example of ceramic material having a positive temperature coefficient (PTC resistors) or having a negative temperature coefficient (NTC resistors) but also in electrical capacitors having a ceramic body of dielectrically-effective material and, in particular, in voltage-dependent electrical resistors (varistors or, respectively, VDR resistors), however, the problem of adequate resistance to thermal cycling, whereby a multitude of brief temperature changes between high temperatures, for example 360.degree. C. and above, and low temperatures, for example -40.degree. C. and below, for example down to -55.degree. C., whereby the different coefficients of expansion of the current leads, on the one hand, and of the ceramic body, on the other hand, cause mechanical stresses which, when the maximum rupturing stress is exceeded, lead to cracks in the ceramic body or even to the destruction thereof is not the only problem, rather the electrical components under discussion here must also have high resistance to surge current loads.
For example, given combination elements composed of one or more PTC resistors (deposit stores) and at least one varistor, surge current loads up to about 2,000 A can occur. Given NTC resistors for limitting inrush current (surge protection NTC), surge stresses having currents of a number of 100 A occur. Similar values can also occur given ceramic capacitors.
Varistors are subject to particularly high surge current loads, these having to be able to resist surge current stresses having currents of 10.sup.3 -10.sup.5 A.
The electrical components under discussion here, particularly the varistors, are thin wafers having a thickness of 0.7 mm-2 mm, thick wafers having a thickness from 10 mm-30 mm and wafers having a thickness lying therebetween. The diameter, particularly of varistors, lies in the order of magnitude of from 30 mm-80 mm.
At their opposite end faces, the ceramic bodies of these components comprise electrodes, usually composed of screen printed silver paste which are applied to the ceramic body in a known manner after it is manufactured and which are heated later at temperatures of between 600.degree. C. and 800.degree. C.
For resolving the problem of resistance to thermal cycling, it would be most beneficial to solder the power leads to these layers in a punctiform manner because mechanical stresses occurring in the temperature cycle would then not have any influence on the ceramic body.
The execution of power leads soldered in punctiform fashion, however, is practically unrealizable for the resistance to surge current. It would be necessary to solder large-area electrodes having good conductivity to the layers in order to achieve a uniform current distribution. When a minimum radius of the power lead is exceeded, the current density at the edge thereof rises above the maximum current density and melting or, respectively, evaporation of the metal of the layer of the component occurs at this location. The maximum current density therefore also depends on the current-carrying capability of the metallized layer. Copper or brass, which are themselves solderable and which can be soldered to the metallic layers of the electrical component, are usually employed as metals for the power supply elements.
Up to a diameter of about 25 mm for the metallic coatings on the end faces of the electrical component, particularly of the varistor, the mechanical stressing still remains below the rupturing stress of the ceramic body given cooling to -40.degree. C. Given larger diameters or greater areas of the metallic layers, the ceramic begins to tear. This especially occurs given thin ceramic layers (for example 0.7 mm-2 mm). The only varistor of this size currently commercially available has a diameter of 60 mm, but the thickness of these wafers is 5 mm. These wafers are contacted by soldering power supply elements to the metal layers. Given thin wafers, particularly given varistors, an adequate resistance to thermal cycling is no longer guaranteed.